I've used it on a really grungy cassette and it worked extremely well. I was cautious not to get it where it could leak into my hubs but soon after I cleaned the cassette I gave the wheel a spin and heard some slight gritty noise. Since I don't have the tools to open up my hub yet I don't know if any damage was done but they seem fine now. It was on a beater bike anyway.
I would say proceed with extreme caution. Also just remove the rear wheel so you don't have to worry about paint or any finishes that might be damaged. It didn't seem to do any damage where it got on the tires either.